This invention relates to a process for polymerizing ethylene. More particularly, it relates to a process for polymerizing or copolymerizing ethylene with a novel Ziegler catalyst at a temperature of 120.degree. C. or more at a pressure of 350 kg/cm.sup.2 or more.
The technique of polymerizing or copolymerizing olefins in the molten state of the resulting polymer at a high temperature under high pressure in the absence of a solvent is called "high pressure ionic polymerization method" and is known.
The high pressure ionic polymerization method is known to be advantageous in that the reactor used is compact and the degree of freedom of selection of comonomer is great. It is also known that such high temperature polymerization is very advantageous in process because a large amount of heat of polymerization can be easily removed during the polymerization and the molten polymer can be utilized as it is in pelletization after the polymerization.
The olefin copolymers obtained by the above polymerization method are used in very many fields such as film, laminate, wire-coating, injection molded article, special molded articles and the like. It is commonly known that in order to obtain products excellent in transparency, impact resistance, blocking properties and the like in each of these uses, the use of polymers having a narrow molecular weight distribution and a narrow composition distribution is beneficial. In particular, in the case of copolymer, as the content of .alpha.-olefin copolymerized is increased the molecular weight distribution and composition distribution affect more greatly the physical properties of the olefin copolymer, and hence, olefin copolymers having a narrow molecular weight distribution and a narrow composition distribution have been desired.
As to Ziegler type solid catalysts for use at high temperatures, various improvements have been made (for example, Japanese Patent Application Kokai Nos. 51-144,397; 54-52,192; 56-18,607; 56-99,209; 57-87,405; 57-153,007; 57-190,009 and 58-203,303); however, the copolymers thus improved have a broad composition distribution and are not satisfactory in transparency and mechanical properties.